Pharmacological comparison of captopril and MK-422 by a new method for measuring activity of angiotensin converting enzyme (ACE)

The authors recently reported the development of a new method for measuring angiotensin converting enzyme (ACE) by means of a highly sensitive angiotensin II RIA technique. We have carried out a comparative study of the pharmacological properties of captopril and MK-422, two ACE inhibitors recently developed as new antihypertensive agents. In this study, in vivo and in vitro animal experiments were performed using the Göttingen Mini-pig (Mini-pig G) animal model of the human disease. In the in vivo experimental system, each drug was administered by intravenous injection at a dose of 1 mg/kg, and a slight difference was found in the time-course of the per cent inhibition of ACE in the blood. In the in vitro system (cultured aortic endothelial cells), the ACE inhibitory activities of the two drugs were compared in terms of the 50%-inhibition point on the dose response curve, and it was found that MK-422 was about 100 times more potent than captopril. These results indicate that our newly-developed experimental system can be useful in the establishment of the clinical dose of vasoactive drugs that act on the renin-angiotensin system.

Angiotensin I converting enzyme activity in hypertension. Relationship to blood pressure, renin-sodium profiles, and antihypertensive therapy

The activity of the angiotensin I converting enzyme was measured in 55 patients with untreated essential hypertension, 11 patients with untreated renovascular hypertension, five patients with untreated primary aldosteronism, and 23 normotensive subjects. Converting enzyme activity was significantly higher (p less than 0.025 or less) in essential hypertension (28 +/- 1 units/ml) and renovascular hypertension (28.5 +/- 3 units/ml) when compared with the activity in the normotensive subjects (21 +/- 1.5 units/ml). Seventeen (31 percent) of the patients with essential hypertension and three (27 percent) patients with renovascular hypertension had an elevated converting enzyme activity above the mean +2 standard deviations value of the normotensive subjects (32.8 units/ml), ranging from 33 to 55.8 units/ml. Converting enzyme activity was similar in black and white patients and in male and female patients, but it tended to decrease with increasing age in both the hypertensive and the normotensive subjects. In the untreated patients with essential hypertension (n = 55), converting enzyme activity was inversely related to mean arterial pressure and age (r = -0.34, p less than 0.01) and positively related to plasma renin activity (r = 0.31, p less than 0.05). Converting enzyme activity was always decreased during captopril therapy, and it was not affected by beta blockers, but it was increased by diuretics. These findings indicate that converting enzyme activity is elevated in patients with essential and renovascular hypertension.

Study of renovascular hypertension in rats. III. Effects of converting enzyme inhibitor (captopril) on the plasma and renal renin activity in two-kidney goldblatt hypertension in rats

Two-kidney, one-clip Goldblatt models were created in Wistar rats by placing a stenotic clip around the left renal artery. A converting enzyme inhibitor, captopril was started just after operation (group 1), 4 weeks (group 2) or 12 weeks after operation (group 3). The plasma and renal renin activity (PRA and RRA) were measured by radioimmunoassay. Group 1 rats, being normotensive during captopril treatment, developed hypertension after cessation of captopril. The PRA was normal, while the RRA of ischemic kidneys was higher than that of the opposite kidneys. In group 3 rats captopril corrected hypertension in 4 of 6 rats. Although after 3 months their PRA was elevated, no difference was found in the renin content of kidneys. Group 2 rats sustained hypertension despite continued captopril treatment. These data suggest that in the early and chronic stage of this type of renovascular hypertension the renin-angiotensin system may play a major role in initiating and maintaining hypertension. In the intermediate stage (group 2), another mechanism may be involved since captopril had no effect while the pattern of the PRA and RRA of group 2 was almost the same as that of group 1.

Left ventricular enzyme activities of the energy-supplying metabolism in Goldblatt-II rats

The hypertrophied left ventricles of renovascular hypertensive Wistar rats were examined for several enzyme activities 4-6 and 8-12 weeks after operation (Goldblatt-II), and compared with controls. The activities of beta-hydroxyacyl-CoA dehydrogenase in hypertrophied myocardial tissue were found to be markedly diminished, as were those of citrate synthase, although to a lesser degree. In both stages of left ventricular hypertrophy hexokinase activity was considerably increased, whereas that of lactate dehydrogenase was only initially slightly elevated. Both enzymes showed an altered isoenzyme composition. The possible reasons and consequences of these changes are discussed.

[Functional activity of the structures of the medulla oblongata in rats with arterial hypertension of renal origin (histoenzymological research)]

A quantitative histochemical study of succinate dehydrogenase (SDH) and NADH-dehydrogenase (NADH-D) activity in medulla oblongata structures was accomplished in rats with arterial renovascular hypertension of the "2 kidneys-2 clips" type lasting 5 months. The systolic arterial blood pressure measured by the tail-cuff method was 179 +/- 4 mm Hg in hypertensive rats versus 108 +/- 3 mm Hg in control. There was a significant elevation of SDH activity in the ventral reticular and commissural nuclei, while in the neurons of the vagus dorsal and ambiguous nuclei it was lowered. NADH-D activity was significantly increased in the neuropil of the hypoglossal nerve nucleus and reduced in its neurons. The general trend was also revealed toward reduction of the maximal and elevation of the minimal activities in other nuclei. These metabolic alterations reflect changes in the functional activity of vasomotor and other structures of the medulla oblongata in renovascular hypertension.

Percutaneous angioplasty in clinical management of renovascular hypertension: initial and long-term results

Between January 1980 and July 1983, percutaneous transluminal angioplasty was attempted on 137 stenotic renal arteries in 100 patients. At termination of follow-up studies (3-39 months, mean of 16 months), 70% of those treated for hypertension had benefited from the procedure. Stenosis secondary to fibromuscular dysplasia responded better than stenosis from arteriosclerosis (85% and 65% of the patients, respectively). Benefit was minimal for those with stenosis of the renal artery ostium or renal insufficiency. Determining levels of renal vein renin before angioplasty is helpful in selecting patients; following angioplasty, this has considerable significance in predicting the success of the procedure.

Renal polyamine metabolism in rats with renovascular hypertension

A standardized stenosis was induced by applying a silver clip around the left renal artery in male rats. This resulted in arterial hypertension within 10 days (as determined by increase in heart weight). Ornithine decarboxylase (ODC) activity was determined in the right (untouched) kidney, the left kidney, and the adrenal glands 1 day, 10 days, and 3 months after the operation. There was no difference in ODC activity in the right kidney of the operated animals when compared with matched controls. In the left kidney (with artery stenosis), ODC activity decreased to 40% after 1 day. A partial recovery was seen after 10 days (ODC activity 70% of normal), and after 3 months ODC activity had normalized. Removal of the clip 1 day prior to killing induced in the 3-month group a more than two-fold increase in ODC activity in the previously clipped kidney; ODC activity in the contralateral kidney was not affected. Only minor changes in ODC activity occurred in the adrenal glands following the operation. Contents of putrescine and spermidine were increased in the left (stenotic) kidney, and after clip removal, also in the right (untouched) kidney. Our observations thus indicate that alterations in renal blood flow are rapidly followed by changes in ODC activity. Contents of putrescine, spermidine and spermine seemed to a great extent to be independent of the ODC activity.

Sensitive direct radio-immunoassay for human renin

A sensitive direct human renin radio-immunoassay has been developed for clinical use. The antigen source was human renal renin purified from Haas' preparation by pepstatin-C6-sepharose affinity chromatography, this was used to prepare a specific human renin antibody. The radio-immunoassay was performed by the double antibody technique using the delayed tracer addition method. Standard curves were obtained over the range 0.2-8.0 ng/ml. Dilution curves of human renal renin and human plasma were superimposable on the standard curve. Both active and inactive renin were detected by this method, and measurements correlated well with total renin activity after trypsin activation. Intra- and inter-assay coefficients of variance were 4.6% and 5.1%, respectively. Renin concentration was higher in patients with renovascular hypertension (1.97 +/- 0.38 ng/ml, mean +/- s.d., n = 10, P less than 0.01), but lower in primary aldosteronism (0.66 +/- 0.16 ng/ml, n = 13, P less than 0.01) compared with essential hypertension (1.38 +/- 0.34 ng/ml, n = 12). This method provides a new tool for the investigation of the renin-angiotensin system in man.

Serum angiotensin converting enzyme activity in normal adults and patients with different types of hypertension

The serum converting enzyme activity (SCEA) was measured in 86 healthy individuals (1.44 +/- 0.82 u, mean +/- SD), 39 patients with essential hypertension (1.53 +/- 0.71 u), 7 patients with hypertension due to renal artery stenosis (1.76 +/- 0.77 u), 14 patients with chronic renal failure (2.10 +/- 0.57 u), 7 patients with renal failure and hypertension (2.62 +/- 0.35 u), 22 normotensive pregnant women (1.02 +/- 0.26 u) and 6 hypertensive pregnant women (1.1 +/- 0.3). No difference was detected between men and women or between normotensives and hypertensives. However, a significant rise in SCEA was found in patients with chromic renal failure (P less than 0.005), in whom an enlarged pulmonary vascular bed and accelerated cellular breakdown are thought to be the causes of the elevated SCEA. During pregnancy, subnormal SCEA was found (P less than 0.005), and this is thought to be due to the enzyme consumption in the kinin system, which is activated during pregnancy. We assume that converting enzyme is not a limiting factor in angiotensin conversion, and most probably it does not contribute significantly to the pathogenesis of hypertension.

Dissecting aneurysm of renal artery. Reversible cause of high-renin hypertension

A forty-two-year-old man was evaluated for malignant hypertension. Renal angiography demonstrated several aneurysmal dilatations and dissections involving the right renal artery. During a period of conservative medical management, hypertension was resolved; repeat renal angiography demonstrated marked spontaneous resolution of the lesion.

[Changes in the activity of renal intracortical kallikrein during induction of Goldblatt arterial hypertension of the 2 kidney--1 clip type in the rat]

Renal tissue kallikrein and proteins were measured in two kidney-one clip Goldblatt hypertensive rats both in the stenotic and the controlateral kidney and in sham operated rats at either 1 or 2 weeks after clipping. Activity was assessed by the amidolytic activity and by the kininogenase activity. Kallikrein in normotensive controls was 97.4 +/- 13 ng of bradykinin min-1 mg-1 of protein at week 1 and increased up to 116 +/- 18. Kallikrein in the GH rats was 83 +/- 12 in the stenotic kidney and 85,6 +/- 14 in the controlateral one at week 1, these values remained unchanged at week 2. As a consequence renal tissue kallikrein became significantly lower in the GH rats only at week 2 when compared to controls both the clipped and unclipped kidney showed the same magnitude decrease. Protein concentration remained at a steady level through out the 2 weeks of study. The results suggest that the lower renal kallikrein activity secondary to hypertension found in GH rats result from a decreased activation of prekallikrein in both kidney.

Chronic captopril infusion in two-kidney, one clip rats with normal plasma renin concentration

Immediately after clipping, plasma renin and angiotensin II concentrations are raised in the two-kidney, one clip rat. Plasma renin and angiotensin II fall to relatively low levels between two and five weeks after operation, while blood pressure continues to rise. An experiment was carried out to determine whether complete suppression of angiotensin II during this two- to five-week period would have any effect on the development of hypertension. Captopril infused chronically (2 mg/kg/h) for up to 20 days completely prevented the rise in mean blood pressure [110.8 +/- 17.9 at day 0 compared to 88.3 +/- 8.4 (s.d.) mmHg at day 20] seen in an equivalent control group infused with dextrose (109.2 +/- 11.6 at day 0 compared to 131.7 +/- 23.6 mmHg at day 20). Preinfusion values for plasma renin and angiotensin II concentration were within the upper half of the range found for sham-operated normal rats. These findings would suggest that while the plasma levels of angiotensin II are low in relation to those seen immediately after clipping, they are still raised in comparison with normal sham-operated rats; this may contribute to the development of hypertension by a slow chronic, rather than an acute, effect.

Central and peripheral catecholamine synthesizing enzymes during the development of two-kidney, one clip hypertension in rats

The activities of the catecholamine synthesizing enzymes have been determined in discrete brain areas and in peripheral tissues of rats, at different times after clipping the left renal artery (two-kidney Goldblatt hypertension, 2KGH) and in sham operated animals. Three days after clipping the only enzymatic change was a slight decrease in plasma dopamine-beta-hydroxylase (DBH) activity. Ten days after clipping no change in enzymatic activity was found at the central level. However, the DBH and the phenylethanolamine-N-methyltransferase (PNMT) activities were increased in the adrenal medulla (+49.0%, P less than 0.001 and +36.6%, P less than 0.001, respectively) and DBH activity was also increased in the superior cervical ganglia (+22.8%, P less than 0.01). These data suggest that sympathetic hyperactivity is present in 2KGH rats when hypertension is established. In addition, as this type of hypertension does not alter the PNMT activity in brainstem areas, it seems that the alterations in PNMT activity reported for genetically hypertensive rats are unlikely to be secondary to the elevated blood pressure.

Plasma kinin levels in acute renovascular hypertension in dogs

The role of kinins in the hypertensive response to acute renal artery constriction (RAC) was examined in the dog. RAC resulted in an increase in systemic arterial pressure (SAP) from 144 +/- 6 to 155 +/- 4 mm HG (p less than 0.05). Simultaneously, arterial plasma bradykinin decreased from 2.3 +/- 0.2 to 1.4 +/- 0.1 ng/ml (p greater than 0.01), while renal venous bradykinin remained unchanged (2.3 +/- 0.2 to 2.0 +/- 0.4 ng/ml, p greater than 0.05). At the same time urinary kallikrein decreased from 55 +/- 6 to 33 +/- 4 milliesterase units (mEU)/min (p less than 0.05), while urinary kinin decreased from 3.2 +/- 0.4 to 1.9 +/- 0.3 ng/min (p less than 0.05). There was a significant correlation between the decrease in arterial bradykinin and the rise in SAP induced by RAC (p less than 0.01). Administration of the dipeptidyl hydrolase inhibitor SQ20881 during RAC reduced angiotensin-converting enzyme levels from 578 +/- 86 to 10 +/- 0.0 mU/ml (p less than 0.005). There was an associated increase in arterial bradykinin (1.4 +/- 0.1 to 5.8 +/- 0.8 ng/ml, p less than 0.001), renal venous bradykinin (2.0 +/- 0.4 to 5.7 +/- 0.5 ng/ml, p less than 0.005), and urinary kinin (1.9 +/- 0.3 to 5.0 +/- 0.7 ng/min, p less than 0.01) in conjunction with return of SAP to control levels. Urinary kallikrein, however, remained depressed following SQ20881 (33 +/- 4 to 30 +/- 5 mEU/min, p greater than 0.05). These results suggest that (1) decreases in circulating BK may potentiate the vasoconstrictor effect of angiotensin II and contribute to the hypertension induced by RAC, and (2) urinary kallikrein is an unreliable marker of changes in plasma bradykinin in this model of hypertension.

Enhancement of renal venous renin ratios by intravenous hydralazine in renovascular hypertension

Renal venous renin (RVR) studies were done in 34 patients with moderate-to-severe hypertension before (unstimulated) and after (15- and 30-minute samples) a 20-mg bolus of i.v. hydralazine. The unstimulated lateralizing ratio of angiographically abnormal kidney or ipsilateral (I) to contralateral (C) RVR of greater than or equal to 1.5 was found in 69%, and the unstimulated ratio of I-inferior vena cava (IVC) renin below renal veins (I-IVC)/IVC (index of reduced renal blood flow) of greater than or equal to 0.48 was present in 50% of 16 patients (10 unilateral and 6 bilateral renal artery stenosis). The I/C and I-IVC/IVC ratios were abnormal in 100% and 88%, respectively, in 1 or both of the posthydralazine sampling in these patients. Hydralazine increased the absolute RVR from the ischemic kidney more than the contralateral kidney and did not result in new false-negative or positive I/C ratios. It is concluded that hydralazine stimulation of RVR is a safe and reliable way to determine the functionally significant pressor kidney in renovascular hypertension.

Studies on the active and inactive renin in renovascular hypertension

Active and inactive plasma renin concentrations (ARC and IRC) from both renal veins and the femoral artery, and the molecular weight (MW) of active and inactive renin (AR and IR) from both renal veins were measured in 6 patients with renovascular hypertension due to unilateral renal artery stenosis. Venous ARC on the affected side was higher than that on the unaffected side and arterial ARC. In 4 patients ARC increased and IRC decreased after circulating through the stenotic kidney, while in the other 2 cases venous IRC on the stenotic side was higher than arterial IRC. The MW of AR on the stenotic and non-stenotic sides were 40000-48000 and 45000-48000, respectively, and those of IR varied from 45000-53000 and 48000-55000, respectively. These values seem to be smaller than those of normal subjects. The renin molecule may be small immediately after the release from the kidney and become larger in the circulation. In 3 cases, whose ARC increased and IRC decreased after passing through the stenotic kidney, AR and IR from the stenotic kidney were smaller than those from the non-stenotic kidney. Some mechanism of AR and IR molecule reduction may work in the stenotic kidney to activate IR.

[Reactivation of renin secretion by a contralateral kidney in the chronic phase of experimental renovascular hypertension]

When one renal artery is markedly narrowed, the renin-angiotensin system (RAS) is only suppressed at the initial stage of vasorenal hypertension in the contralateral kidney, and is reactivated during the chronic stage as manifested in normalized or increased total tissue content of active renin, its enhanced secretion and its activity in the outflowing blood. This kidney's RAS begins to have a prevailing effect on systemic RAS, while that of the ischemized kidney makes a smaller contribution. Similar changes can be observed in some animals with a moderately narrowed renal artery.

Regulation of angiotensin converting enzyme

Angiotensin converting enzyme (ACE;EC 3.4.15.1), or kininase II, was studied in serum, cultured endothelial cells from cord artery, in macrophages of humans, and in serum and purified plasma membranes of rats following treatment with inducers of ACE biosynthesis. ACE activity was measured in biological fluids with an enzyme kinetic method employing synthetic 1-hipp-1-his-l-leu tripeptide as a substrate, and with a new method using 125I-labelled specific inhibitor of ACE as a sensitive probe for ACE binding sites. The latter technique also proved suitable for the quantification of ACE in cells. Anti-human ACE antibody was employed for immunofluorescence studies in human cells. Dexamethasone treatment caused an increase in ACE in cultured human endothelial cells, macrophages and in rat pulmonary plasma membranes, but failed to increase serum ACE activity in rats. Captopril and enalapril treatment of hypertensive patients increased total serum ACE, the increase being evident after removal of the active drug from the serum by prolonged storage or chloramine T treatment (captopril) or by dialysis (enalapril). Captopril increased the ACE content of endothelial cells and macrophages. Macrophages appeared sensitive to captopril induction of ACE biosynthesis after pre-stimulation with Escherichia coli lipopolysaccharide. Dexamethasone treatment potentiated the known induction of ACE in rat pulmonary tissue. Thus ACE biosynthesis may be enhanced by three categories of treatment: (1) glucocorticoid; (2) macrophage activation; (3) ACE inhibitors. The precise mechanism of ACE induction and its possible biological relevance await further clarification.

Uptake of inactive renin by human ischaemic kidney

1. Plasma samples from both renal veins and infrarenal inferior vena cava were studied in 21 patients with unilateral renal artery stenosis and 11 patients without significant renal artery stenosis (essential hypertension), both groups being on unrestricted sodium intake.

2. Whether inactive renin was activated by acid or trypsin, it tended to be increased by 50–100% in unilateral renal ischaemia patients compared with essential hypertension. Active renin was increased two- to four-fold in unilateral renal ischaemia patients compared with those with essential hypertension. Thus the ratio of active to total renin in peripheral venous blood tended to be higher in unilateral renal ischaemia (0.37) than in essential hypertension (0.30) patients.

3. in renal vein blood from the affected kidney in unilateral renal ischaemia, the proportion of active renin to total renin was very high (68% for trypsinized samples and 73% for acidified samples). When affected renal vein blood was compared with infrarenal vena caval blood (equivalent to renal artery blood in amount and proportions of active and inactive renin), there was found to be a marked increase of active renin (ratio 2.83, significance of difference from 1.0, P < 0.001), as expected.

4. Inactive renin was decreased in affected renal vein blood compared with peripheral blood (ratio of renal vein to peripheral blood renin). This occurred whether acidification was used (ratio 0.62 ±0.11, P < 0.01) or trypsin (ratio 0.70 ± 0.13, P < 0.05).

5. The cause of the apparent uptake may be conversion of inactive renin into active renin by protease action in the ischaemic kidney. Urinary excretion or lymphatic drainage would seem unlikely.

Surgical reversal of renovascular hypertension in rats: changes in blood pressure, plasma and aortic renin

1. Blood pressure, plasma and aortic renin concentrations were measured in Goldblatt two-kidney one-clip hypertensive rats before and after surgical correction by removing the renal artery clip. 2. Blood pressure fell rapidly in the first hour and then more slowly over 24 h. 3. Plasma renin concentration fell into the normal range by 3 h after unclipping. 4. Aortic renin concentration was markedly raised in hypertensive rats and declined slowly after unclipping, being virtually unchanged at 3 h and reaching normal levels at 24 h. 5. The fall in blood pressure produced by removal of the renal artery clip in Goldblatt two-kidney one-clip hypertensive rats does not depend upon the renin-angiotensin system in plasma and vascular tissue and indicates that a renal vasodepressor system may be involved.

The importance of the renin-angiotensin system in the development and maintenance of hypertension in the two-kidney one-clip hypertensive rat

1. Blood pressure, renin concentration and angiotensin II were measured in unanaesthetized two-kidney one-clip hypertensive rats at 1 and 2 days, at weekly intervals up to 10 weeks and at 15 and 20 weeks after clipping. 2. Compared with values in sham-operated rats, renin and angiotensin II were initially increased at 1-2 days but were then suppressed between 2 and 4 weeks to levels similar to that found in sham-operated rats. Between 5 and 20 weeks renin and angiotensin II increased again to high levels. 3. There was a significant correlation between angiotensin II and blood pressure in acute rats 1-2 days after clipping (P less than 0.05) and in chronic rats 8-20 weeks after clipping (P less than 0.001). There was no difference in the slope of the regression lines but the regression line for the chronic rats was shifted upwards in a parallel manner. 4. The acute hypotensive response (-20.3 +/- SD 24.9 mmHg) in 26 chronic rats given converting enzyme inhibitor was related to the basal renin and angiotensin II levels and followed the slope of the angiotensin II/blood pressure regression line for all chronic rats. Only one out of 26 rats reduced its blood pressure to normal levels. 5. In 12 rats at 4 weeks after clipping, when blood pressure was elevated but angiotensin II was suppressed, there was only a small fall in blood pressure (-7.1 +/- SD 7.2 mmHg). This also followed the angiotensin II/blood pressure regression line for chronic rats but at the lower end. Blood pressure again was not reduced to normal. 6. These results suggest that renin and angiotensin II are increased up to 20 weeks after clipping, that there is no change in the net vascular responsiveness to endogenous angiotensin II at any stage in this experimental model and that the acute effect of angiotensin II is determined solely by its position in the same dose-response curve. Also with the exception of 1-2 days immediately after clipping the acute effect of angiotensin II plays only a minor, though variable, role in the hypertension and that some other mechanism, as yet undetermined, is of greater importance and begins to have an effect as early as 2 weeks after clipping.

Measurement of renin in segmental renal veins of hypertensives

In 27 of 270 patients studies for possible renovascular hypertension, a lesion affecting a segmental renal artery was seen on angiography. In 22 patients who had sufficient data, the significance of measuring segmental-renal-vein renin in the diagnosis and treatment of segmental disease was evaluated. In 11 (50%), the renin values of segmental vessels supported the clinical diagnosis. In four (36%) of these 11, a high-renin-release area would not have been identified without the stress of upright posture. Successful treatment of hypertension was achieved in six of nine patients who had a revascularization procedure or resection of the segmental vessels. Samples from involved and uninvolved segments of the kidney as well as from the contralateral renal vein and inferior vena cava, together with the stimuli of sodium and volume depletion augmented by upright posture, enabled confirmation of the angiographic diagnosis of segmental-renovascular hypertension.